GLOSSARY
Photon--a quantum of electromagnetic radiation, equal to Planck's constant multiplied by the frequency in hertz. PA0 Normal Metal--Any metal not in a superconducting state, e.g. silver, gold, copper. PA0 Quasi Particle--fundamental energy excitation of a super-conductor similar to an electron PA0 Microbolometer--device for measuring very small energy levels of microwave and infrared energy. PA0 Tunnel junction--an electron device that allows quantum mechanical tunneling of electrons through an insulating barrier whose thickness is a few nanometers. PA0 Particle--any object which has a quanta of energy that can be absorbed by the calorimeter. examples are: optical, ultra violet, x-ray, and gamma ray photons; proton, neutron and alpha particles; ions, and neutral atoms; molecules; and phonons. PA0 Phonon--a quantum of vibrational energy of atoms in a solid, equal to Plank's constant multiplied by the frequency in hertz. PA0 Semimetal--metal such as bismuth with very low conduction-electron density. PA0 Superconductor--certain metals, alloys, and compounds in which the resistance drops essentially to zero below a critical temperature near absolute zero.
Radiation detectors are crucial components of many commercial and scientific measurement apparatuses. Because of the wide use of radiation detectors, there has been much development of these kinds of detectors to measure the many different types of radiation and to measure them more accurately. Although the invention discussed here can be used as a general purpose radiation detector, we will initially describe the detection of X-rays since this is the most immediate application of the present invention. Our x-ray detector gives more accurate energy resolution and higher speed than other competing technologies.
Every element in nature emits X-rays with a characteristic or a set of characteristic energies. When an X-ray is detected and its energy accurately determined, then one can simply infer from the energy the constituent element it came from. Typically one measures an unknown sample with many different constituent elements. The x-rays and their energies collected from the unknown sample is then displayed according to energy in an x-ray spectrum, from which the constituent elements can be deduced. This basic idea is behind a large class of analytical measurement instruments that serves a wide variety of industries ranging from mining (ore composition) to semiconductor fabrication (composition and contaminant determination).
Energy resolution of the x-ray detector is a major specification for these type of x-ray radiation detectors. The better the resolution for the detection of x-rays, the more reliably one can "tag" that particular x-ray with an element. Because of other imperfections and background x-ray signals that are always present in a measurement system, improved x-ray energy resolution also allows one to better determine how much (the percentage) of a given element is present in an unknown sample.
It is always useful to take an x-ray spectrum with as many x-ray events (or counts) as possible. This makes the "signal" of the x-rays as large as possible as compared with the background "noise" that is always present in a real measurement. The capability to take a spectrum with more counts are generally given through two other important specifications. One is the area of the x-ray detector. Since the detector always has to be located at a finite distance away from the sample, it can intercept only a fraction of x-rays that are emitted from the sample. Increasing the area of the detector is thus desirable as it subsequently increases the number of x-rays that can be detected. The last important specification is the maximum permissible count rate of the x-ray detector. One typically operates these instruments by taking an x-ray spectrum such that it gives a fixed number of total counts. The spectrum is then accumulated over a time from typically several minutes to hours. A faster count rate allows the x-rays to be collected over a smaller time period. Improvements in speed can be very important for real applications. For example, a factor of 100 increase in maximum count rate reduces a measurement time of 10 hours to one that only takes six minutes. This large decrease in measurement time is obviously very important for scientific and commercial applications.
Commercial instruments today are primarily based on two detector technologies. The most widely used is called an Energy Dispersive Spectrometer (EDS) which uses a crystal of silicon cooled to 77 Kelvin. At low temperature the electrons in the silicon are frozen into their atomic positions and no electrical current flows. However, when an x-ray interacts with the silicon it breaks the atomic bond of an electron and allows it to freely flow through the crystal. An amplifier then measures the electrical current from these electrons moving through the crystal, with the magnitude of the current being proportional to the energy of the x-ray that interacted with the crystal. (The higher the x-ray energy, the more electrons are freed from their positions, giving more electrical current.) This technology is now quite mature and has leveled out to a resolution of about 130 eV (electron volts), and maximum count rate of about 3 thousand/second, and a collection area of 2-4 mm.sup.2.
CHART I ______________________________________ SUMMARY OF ENERGY DISPERSIVE SPECTROMETER (EDS) ______________________________________ Method of Operation X-ray hits a sensor and creates an electric current Resolution 130 eV for 5000 eV X-ray Sampling Speed 3,000-10,000 counts per second Collection Area 2-4 mm.sup.2 Operating Temperature Usually 77k, liquid nitrogen ______________________________________
Wavelength Dispersive Spectrometers (WDS) constitute a second type of detector technology which diffract x-rays from a crystal at angles that depend on the x-ray energy. These detectors have a good energy resolution of approximately 5 eV, but because the effective collection area is very small and its cost is high, only about 5% of the instruments are based on this detector.
CHART II ______________________________________ SUMMARY OF WAVELENGTH DISPERSIVE SPECTROMETER (WDS) ______________________________________ Method of Operation X-ray reflects off a crystal in a measurable angle Resolution 5 eV for 5,000 eV x-ray Sampling Speed To 50,000 counts/sec Collection Efficiency Collection efficiency 1-10% of EDS method Operating Temperature Ambient (300 kelvin) ______________________________________
A more ideal detector would have the ease of use and large collection area of the EDS detector, but with the high resolution of the WDS type system. Since the beginning of the 1980's, there has been work to try to reach this goal with new type of x-ray detectors that operate at very low temperatures (less than 4 kelvin). Although many approaches have been investigated, summarized below are two approaches that have achieved resolution better than 100 eV, a resolution that at least becomes competitive with the present types of EDS systems.
One type of detector uses superconducting-insulator-superconducting (SIS) tunnel junctions. The principle of operation of this device is similar to the EDS type detector, although here the way the current is generated through the device is physically different. In a superconductor, the electrons are bound into "Cooper pairs" which form the superconducting state. An x-ray interacting with the superconductor then breaks these Cooper pairs and forms excitations of the superconductor called quasiparticles. These quasiparticles are analogous to the free electrons which are produced in the EDS system when electrons are freed from their atomic positions. The method of collection of the quasiparticles differ from the EDS detector. Here, a superconducting "tunnel junction" is placed in the region where there are quasiparticles. The quasiparticles quantum mechanically tunnel through the junction, which is then registered as an electrical current. Although the basic principle of this detector has been well known, the experimental challenge is to ensure that most of the quasiparticles tunnel through the tunnel junction. Various important improvements have been made in the geometry of a device by Boothe and by other workers to force the quasiparticles to diffuse and then trap themselves in a region near the tunnel junction. The latest work to improve tunneling is disclosed in U.S. Pat. No. 5,321,276 (1994) to Kurakado et al.
Another type is the x-ray microcalorimeter as first reported by Moseley et al. as noted in the NASA experiments described below. This device is based on a very simple and well known calorimetry principle where energy is absorbed and then converted into heat. For this x-ray detector, the subsequent temperature rise of the heat is proportional to the x-ray energy. Because the actual x-ray energy is quite small, the heat capacity of the detector has to also be very small so that a relatively large temperature rise is observed. This is accomplished by operating the device at very low temperatures (less than 0.3K), making the detector small, and by having the detector made out of non-metallic materials which, at low temperatures, have much lower heat capacities than metals. The detectors are typically made out of micro-machined silicon which is an insulator at low temperatures. The thermometer is made from doping a part of the silicon so that it just slightly conducts electrical current at these low temperatures--this doping makes the electrical resistance temperature dependent. Improvements in the detector have improved on initial performance greatly. The improvements have been to use separate x-ray absorbing elements, made from a superconductor or semi-metal which have very low heat capacity, and then connecting these absorbers to the main body of the micro-machined silicon in such a way that the heat transfer is accomplished in a uniform way across the body of the absorber. At present, the resolution of these detectors are very good, 8 eV for the best device and typically 12-14 eV is found. However, one major drawback of this detector is it slow speed. Typical fall-time response of the thermal pulses is 1 to 10 mS. Although this is adequate for some scientific applications, this is much slower than desired for commercial applications.
The invention described here is based on the micro-calorimeter idea. Although when reduced to practice the invention uses a superconducting-insulator-normal metal tunnel junction, it is not related to the superconducting-insulator-superconducting tunnel junction detectors discussed above: our detector measures the heat rise in the normal metal, not the presence of superconducting quasiparticles as in the latter case of the Booth et al. (Applied Physics letters, Vol. 50, No. 5, pg 293-295, 1987) experiments.
Summarized below is all the known relevant prior art including the publication source.
Thermal detectors as x-ray spectrometers Moseley, Mather and McCannon, J. Appl. Phys, 56(5), Sep. 1, 1984 teaches an apparatus of the microcalorimeter type to measure the very small energy pulses of x-ray photons striking a silicon target. By 1994 these NASA experiments had achieved the extremely high resolution of 8-12 eV. The experiments run at 0.1K. However, the method and apparatus is very slow even compared to commercially available instruments. The NASA experiments are only achieving speeds of recording 100 events per second as compared to industry standards of 10,000 events per second. Thus, for lab use, a spectrometric measurement could take several hours versus several minutes.
The most recent experimental apparatus uses a Pb target for the x-rays. A thermal connection exists to a layer of silicon which is suspended in a vacuum supported by legs. A thermistor implant is imbedded in the layer of silicon. The thermistor current is measured through leads of superconducting A1 carried in the support legs. The silicon is an insulator having low heat capacity. Since .DELTA. Temperature=.DELTA. Energy/C, where C is the Heat Capacity, the low heat capacity of the silicon accounts for the extremely high resolution of the device. These experiments are the closest known relevant prior art to the present invention.
Design Analysis of a Novel Hot-Electron Microbolometer, Nahum and Richards, IEEE Transactions On Applied Super-conductivity 3, 2124 (1993) teaches the use of super-conducting antennas to measure the infrared radiation. The rf current from the superconducting antenna is dissipated into a resistive metal strip. The resulting temperature rise of the electrons in the metal strip are measured as a change in the voltage across a normal metal-insulator-superconductor tunnel junction which is biased at a constant current.
U.S. Pat. No. 4,869,558 to McDonald discloses a superconducting device operating below the transition temperature having multiple layers of thin film. The device operates on the temperature dependent inductance with highest sensitivity when at least one of the layers is thin relative to the magnetic penetration depth of the superconducting material utilized.
U.S. Pat. No. 4,739,382 to Blouke et al. discloses a charge-coupled device package used for temperature sensing. This sensor operates at LN.sub.2 temperature.
U.S. Pat. No. 4,943,559 to Severin et ai. discloses a temperature sensor comprising a thin layer of superconducting material the thickness of which varies over its length. The Tc of the material is portional to its thickness. The temperature of the sensor is derived by the electronics controlling the bias temperature of the sensor.
U.S. Pat. No. 5,090,819 to Kapitulik discloses a superconducting bolometer which derives temperature by controlling the temperature of a superconductor at the midrange of the superconductivity/non-superconductivity transition region and measuring the resistance change due to impairing radiation.
U.S. Pat. No. 5,171,733 to Hu discloses a bolometer formed by a high TC superconductor and an antenna.
U.S. Pat. No. 5,179,072 to Bluzer discloses a multispectral super-conductive detector using geometric and kinetic inductances.
U.S. Pat. No. 5,321,276 to Kurakado et al. discloses a superconductor-insulator-superconductor tunnel junction radiation sensing device includes first and second superconductor electrodes and a tunnel barrier layer interposed therebetween. The tunnel barrier layer is made up of a thin-wall portion and a thick-wall portion each formed of a superconductor or an insulator, and each having opposite surfaces respectively contacting the first and second superconductor electrodes, and each extending adjacent each other in a same horizontal plane between the first and second electrodes. The invention improves the Booth tunnel junction design of wide and narrow super conductor plate combinations.
The difference in our invention and previous micro-calorimeters is the essential use of normal metal in the thermal circuit of the calorimeter. The present invention measures the temperature rise of the electrons in the normal metal, as opposed to previous experiments which measured the temperature rise of an insulating material where the heat was carried by vibrations of the atoms in the insulator (these vibrations are call phonons). Experiments have generally not tried to use a normal metal in the detector because it is well known that metals have large heat capacities. Thus, it was generally believed that their inclusion would reduce the thermal signal of the detector. We show in this invention that their use is quite beneficial to the circuit as long as small amounts of metal are used and are placed properly in the circuit. The performance of the circuit is dramatically improved while not increasing the heat capacity of the metal to unacceptable levels. The most important improvement is that the operation speed is increased by a factor of about 100 over Moseley al.